Hello, I'm new in java, and I have a problem. In this class I make a query to the database and return the result but this not return nothing.
When I call the method in the main program, it return an exception that says java.lang.NullPointerException.
This is the code of the class and the main program:

@Name("Actividad")
public class Actividad extends EntityQuery {

     private static final long serialVersionUID = 1L;

     private static final String[] RESTRICTIONS = { };
     
     @Override
     public String getEjbql() {
          return "select campo from NmcampoPlantilla campo";
     }
     
     @Override
     public List<String> getRestrictions() {
          return Arrays.asList(RESTRICTIONS);
     }
     
     @In     EntityManager entityManager;
     @SuppressWarnings("unchecked")
     public List<String> camposActividad(){
               
          return entityManager.createQuery("select actividad from Nmactividad actividad, Nmplantilla plantilla where actividad.idListaPlantilla=plantilla.idListaPlantilla order by actividad.actividad").getResultList();
          }
     
}

This is the main program:

public class probarActividad {

     public static void main(String[] args) {
          try{
          
          List<String> a = new ArrayList<String>();
          
          Actividad act = new Actividad();
          act.camposActividad();
          }
          catch (Exception e){
                System.err.println(e);
                System.exit(0);
              } 
     
     }     
}

The method camposActividad() the method should return a List of object.

Can anybody look the code and helpme?

Thanks